400 University of California Publications in Geology [Vol. ll 



cate. Above the mixture thus produced a solution of ammonium car- 

 bonate was poured, taking care to avoid mixing of the ammonium 

 carbonate solution with the suspension of clay. As the ammonium car- 

 bonate diffused down it produced a flocculation of silicic acid, and 

 in the formation of the gelatinous silica a banding resulted, consist- 

 ing of alternate layers of fine clay, and clear silica jelly (plate 36). 

 Evidently as the silica assumed the gelatinous state it was able to 

 force the clay aside. 



The bands varied somewhat in width in different experiments. 

 They were approximately equally spaced, and while fairly regular, 

 they showed all the minor structures which have been described as 

 seen in the bedding of chert. They showed lenslike terminations of 

 both clay bands and silica bands ; they showed an irregular thicken- 

 ing and thinning. Some nodules were produced. 



This separation of gelatinous silica into bands occurred not only 

 with clay but with other substances. The experiment was modified 

 somewhat by using some of the red shale partings between chert 

 beds in the Franciscan, powdered to pass a 100-mesh screen. This 

 powder was suspended in a solution of sodium silicate. The coarser 

 particles settled to the bottom, and the supernatant liquid, containing 

 much fine suspended shale, was siphoned off, leaving the mass at the 

 bottom, consisting of incoherent shale particles permeated with a 

 solution of sodium silicate. Over this, a solution of ammonium car- 

 bonate was carefully poured and diffusion allowed to go on as before. 

 There resulted a series of bands of clear silica jelly and red shale, 

 which duplicated many of the features of the chert and shale beds. 



In another experiment, pure white crystalline quartz was pow- 

 dered to pass a sixty-mesh screen. This was put into a solution of 

 sodium silicate and after the coarser particles had settled to the bot- 

 tom the supernatant liquid was withdrawn, leaving a mass of fine 

 quartz grains permeated by a solution of sodium silicate. After pour- 

 ing ammonium carbonate over this mass and allowing it to stand for 

 some time it was found that a sharp banding had resulted, giving 

 clean cut, lenslike bands of quartz sand interbedded with layers of 

 silica jelly. 



In all these instances the number of bands which would form 

 seemed to be limited. After a few bands were produced in the upper 

 part of the silica suspension, the gelatinous silica formed around the 

 particles of clay, shale or quartz without further segregation, as 

 shown in the photographs (plate 36). 



